1. Field of the Invention
This invention relates to test equipment used to measure the flying height of a read/write transducer of a computer disk drive storage device, and more particularly to a method and apparatus for calibrating an instrument which determines the flying height of a read/write transducer head over a rotating disk of a disk drive.
2. Description of Related Art
Disk drive devices are well established in the computer industry as a means for storing data. Disk drives include a rotating disk on which information is written and read by a "read/write" transducer head suspended over the rotating disk. It is common today to design the head of a disk drive such that, during operation, the head is suspended by an air cushion created by the relative motion of the disk with respect to the head. The cushion of air exerts an upward force on the head that permits the head to "fly" over the disk as the disk rotates underneath the head. Because the height at which a head flies over a disk is important to the operation of a disk drive, disk drive head manufacturers must be able to determine the "flyability" of the heads which they produce (i.e., the upward force exerted by the cushion of air that flows between the head and the disk), and the height to which that force will raise the head above the disk. Purchasers and manufacturers of disk drives often evaluate the performance of a disk drive based upon the flying height of the head over the disk during operation.
Presently, the three most widely used methods for determining the flying height of a head of a disk drive during operation are: (1) capacitance method; (2) optical interferometer method; and (3) laser Doppler vibrometer method. In each method, a head is flown over a rotating disk.
In the capacitance method, the distance between the head and the disk is determined during the flight of the head by applying the formula C=E.sub.o (A/d); where A is the area of a projection of the head onto the disk, projected in a direction perpendicular to the plane of the disk; E.sub.o is the dielectric constant of air; and d is the distance between the head and the disk. The dielectric constant of air is known, the area of the projection of the head onto the disk is easily determined, and the capacitance can be measured by a variety of well known methods. Thus, the distance d can be determined. When the capacitance method is used, a special head and disk are required to determine the capacitance between them.
In the optical interferometer method, variations of the fringe position or the intensity of the Newton's rings are used to determine the height of the head over the disk. For this method, a transparent head or disk is used.
The laser Doppler vibrometer method measures the velocity of the head by measuring the frequency shift of reflected light from a special vibrating test head.
Regardless of which of these three methods is chosen, the apparatus that is used to determine the flying height must be calibrated. Calibration of the apparatus identifies a correction factor which compensates for all variables in the process or apparatus, such as differences between the gain of different systems, variations in materials, etc. The correction factor is applied to the calculated flying height of each subsequently measured head to determine the actual flying height of each head. In order to calibrate the measurement instrument, the actual flying height of a head must be determined by a method that is independent of the measurement instrument. The need to know the actual flying height of a head prior to the calibration process presents an obvious dilemma; that is, how to determine the flying height of a head so that the apparatus that measures the flying height of a head can be calibrated.
Currently, the most widely used method to calibrate a flying height measurement instrument uses a disk having protrusions, or "bumps". The height of each bump can be measured to a very high degree of accuracy. The flying height of the head is measured by the method to be calibrated (e.g., the optical interferometer method). Then, the true flying height of the head is determined by passing the head over a bump disk. Contact between the head and a bump is detected by one of at least three methods: (1) electrical resistance technique, (2) acoustic emission technique, or (3) piezoelectric transducer (PZT) technique. The PZT technique is the most widely used of these methods. When the head comes into contact with a bump, a piezoelectric transducer (PZT) mounted on either the bump or the head generates an electric voltage. The level of the voltage produced is proportional to the velocity of the disk and the relative height of the head.
A problem with the PZT bump disk method and apparatus for calibrating a flying height measurement instrument is that factors other than the velocity of the head and the flying height of the head can also influence the output of the PZT. These factors include: the shape of the bump (in particular, the surface contour of the bump); the relative attitude of the head (i.e., position of each of three orthogonal axes through the head) relative to the bump; and the asperity of the air bearing surface of the head. These factors make the determination of the true flying height of the head imprecise. Similar problems arise when the acoustic emission method of detecting contact between the head and the bump is used. The electrical resistance method has a high sensitivity, accuracy, and repeatability. However, the electrical resistance technique requires a specially made head and disk. Therefore, the head used for calibration of the flying height measurement instrument is not the head whose flying height is to be measured. Therefore, differences between the characteristics of the calibration head and the head whose flying height is to be measured can affect the accuracy of the calibration.
Furthermore, since the bump method is destructive to the head, a special head must be used. Therefore, the flying height measurement instrument cannot be calibrated with the actual head whose flying height is to be measured. This means that even when the PZT technique is used, differences between the actual head to be measured and the calibration head may result in calibration errors.
Since the calibration procedure is imprecise, the true flying height of a transducer head cannot presently be precisely known. Therefore, while the present instruments for measuring flying height can be calibrated to yield the same result each time, they cannot be calibrated to agree with results from other manufacturers' instruments. To overcome this problem, some manufacturers presently establish an offset number or correlation number that is used to convert their results to agree with the results from other manufacturers' instruments. The value of the offset number depends upon the manufacturer of the head to which the head under test is to be compared.
Thus, the prior art means for calibrating a flying height measurement instrument has the following problems: (1) the output of the PZT method depends on the velocity, relative height, surface shape of the bumps, attitude of the head, and the asperity of the air bearing surface of the head, causing variations in the output of the PZT and thus calibration errors; (2) the prior art method of calibrating the flying height measurement instrument is destructive to the head used and to the bumps on the disk due to contact between the head and the bumps, hence the life of the calibration apparatus is limited and the cost of the calibration process is increased; (3) since this calibration process is destructive to the head, an actual head cannot be used during the calibration process, thus potentially introducing additional variables which may lead to calibration errors; (4) very tight control of the cleanliness, the shape, and the height of the bumps must be maintained.
Therefore, it would be desirable to have a method and apparatus for calibrating a flying height measurement instrument that: (1) precisely calibrates the flying height of a transducer head, as measured by a measurement instrument, to the actual flying height of the head, (2) is inexpensive, (3) permits a head whose flying height is to be measured to be used during the calibration procedure, and (4) is nondestructive to the calibration apparatus. The present invention provides such a method and apparatus.